This invention relates in general to sewing machines and in particular to a new and useful rotary hook construction for such sewing machines.
A rotary hook construction can be seen from German Pat. No. 409,771. In the known rotary hook thread uptake capacity is to be increased, combined with good sewing properties at high sewing speed, in that the pin of the bobbin case holder is arranged displaced downward relative to the axis of the guide rib of the bobbin case holder and at the same time also laterally by cutting away a part of the sidewall of the bobbin case holder. Due to the eccentric arrangement of the bobbin case, obliquely at the lower right as seen in a front view on the rotary hook, the bobbin can be made larger than conventional bobbins in radial as well as in axial directions. The eccentric arrangement of the bobbin case is intended further to permit sufficiently small edge angles between the bevels on the front and back of the bobbin case holder, thereby facilitating the spreading of the needle thread loop as it is being passed around the bobbin case holder and the bobbin case.
This rotary hook, as it has been known in the literature for many decades, wherein normally conflicting properties, namely great rotary hook thread uptake capacity as well as good sewing properties at high sewing speed are optimized in an apparently ideal manner, but evidently does not have the desired good sewing properties after all, since it has not been proved in practice. In fact, for the modern high speed sewing units with automatic work feed control the machine down times caused by bobbin change are especially disrupting. Since the need for a double lockstitch rotary hook with great thread uptake capacity combined with good sewing property at high sewing speed is nowadays especially great; so far entirely different methods, but also unsatisfactory ones in the last analysis, have been followed to achieve this objective.
Increasing the size of the normal rotary hook as a whole is known. But this leads to the disadvantage that the rotary hook shaft must be placed lower, and that a thread feeder of a different characteristic, and possibly a modified case lifter and a different thread cutting device must be used. These consequent measures, however, conflict with the general trend toward reduction of diversity and the application of the principle of modular construction. Moreover, the larger rotary hook pulls more needle thread under the throat plate as the needle thread loop is being laid around the bobbin case holder than is the case with normal rotary hooks. The result is that the individual sections of the needle thread are pulled through the eye of the needle much more often and hence there is much more stress on the needle thread than in normal rotary hooks.
The rotary hook for double lockstitch sewing machines known from German OS No. 29 53 743 (U.S. Pat. No. 4,284,017) is based on an entirely different principle for enlarging the rotary hook thread uptake capacity. For this rotary hook one uses instead of a two-part bobbin case housing a one part pot type bobbin holder with a central pin for direct uptake of the bobbin. The bobbin fills the entire cavity of the bobbin holder, owing to which its diameter is increased by twice the wall thickness of the eliminated bobbin case. The front flange of the bobbin is made convex and forms the end wall of the rotary hook. The bobbin is retained axially either by a spring loaded lever which is disposed at the hollow pin and applies against the front flange, or by permanent magnets inserted in the bottom of the bobbin holder. The threaded tension at the rotary hook thread is produced by the friction between the bobbin and the rotary hook surfaces surrounding the bobbin on the end face, that is, in one case by the bottom of the bobbin holder and in the other additionally by the lever.
The increased rotary hook thread uptake capacity and simplified construction of this rotary hook are offset, however, by the major disadvantage that as the amount of thread on the bobbin diminishes, the thread tension on the rotary hook thread increases continuously; so that, because of the continually varying ratio between the needle thread tension and the rotary hook thread tension, a different stitch pattern results on relatively long seams.